Synthetic polymeric fibers that have physical and morphological characteristics generally similar to pulp fibers produced from natural woods have been known for approximately 10 years. Examples of such fibers are the synthetic wood pulp fibers formed of polyethylene that are sold by Crown Zellerbach under the trademark SWP.
Various methods of making synthetic wood pulp fibers are known, including (1) solution polymerization accompanied by stirring, (2) dissolving a preformed polymer and subjecting the solution of an anti-solvent, or (3) forming the polymer at the interface between liquid layers, with localized stirring provided to pull the polymers thus formed into fibrillated forms. Examples of methods of producing synthetic wood pulp fibers are disclosed in U.S. Pat. Nos. 3,560,318; 3,081,519; 3,003,912; 3,068,527; and 3,290,207; South African Pat. No. 697,431; and United Kingdom Pat. No. 1,102,342; and Netherlands Patent Application A132/48-7313173.
As used in this specification and the appended claims, the term "synthetic wood pulp fibers" means synthetic, water dispersible, thermoplastic, elongated, supple, randomly bent, polymeric fibers or fibrils generally similar in length and denier to conventional wood pulp fibers produced from naturally occurring woods. Each such "synthetic wood pulp fiber" is of irregular cross sectional shape measured at any given point along its length, and in addition is nonuniform in cross section along its length. The predominant shape of the fibers is usually rather ribbon-like.
Though synthetic wood pulp fibers are similar in length and denier to conventional wood pulp fibers, their uniformity is better and their size and shape consistency greater. Whereas, conventional wood pulp fibers have a length which varies from 0.5 mm to 5.0 mm and a coarseness of between ten and twenty decigrex synthetic wood pulp fibers have a length of from one to four millimeters and a coarseness of between three and ten decigrex (as disclosed, for example, in the section headed "Fiber Dimensions" in the Sept. 1974 publication by Crown Zellerbach entitled SWP.
Comparing some of the other properties of synthetic wood pulp fibers with conventional wood pulp, a conventional wood pulp fiber has from 1 to 5 times the breaking stress of synthetic wood pulp; whereas, the synthetic wood pulp fiber elongates at rupture 3 to 5 times as much as a conventional wood pulp fiber, hence, the overall toughness of a synthetic wood pulp fiber is similar to or even greater than that of a conventional wood pulp fiber (as disclosed, for example, in the section headed "Single Fiber Stress-Strain Behavior" in the September, 1974 publication by Crown Zellerbach entitled SWP).
The present invention utilizes synthetic wood pulp fibers in a high loft, low density, nonwoven fibrous material such as an air-laid web or fabric. Nonwoven materials are structures which consist of an assemblage or web of irregularly arranged fibers, joined randomly or more or less systematically by mechanical, chemical or other means. These materials are well known in the art, having gained considerable prominence within the last twenty years or so in the consumer market, the industrial commercial market and the hospital field. For example, nonwoven materials are becoming increasingly important in the textile and related fields, one reason being because of their low cost of manufacture for a given coverage as compared to the cost of more conventional textile fabrics made by weaving, knitting or felting. Typical of their use is the production of hospital caps, dental bibs, eye pads, dress shields, shoe liners, shoulder pads, skirts, hand towels, handkerchiefs, tapes, bags, table napkins, curtains, draperies, and the like. Generally speaking, nonwoven materials are available today in a wide range of fabric weights of from as little as about 100 grains/sq. yd. to as much as about 4,000 grains/sq. yd., or even higher.
A number of processes and types of apparatus are known for producing nonwoven materials. These include (1) mechanical techniques (e.g., carding or garnetting), (2) wet-laying techniques (e.g., inclined wire paper apparatus, cylinder paper apparatus, etc.), and (3) air-laying techniques. The high loft, low density, non-woven materials such as webs or fabrics to which this invention relates may suitably be produced, in the manner to be explained in detail below, from layers of material manufactured by well-known air-laying processes.
The product of this invention utilizes synthetic wood pulp fibers in a novel high loft, low density, nonwoven, two-ply fibrous material that is useful as one component of an infant's disposable diaper. The material is also useful in other absorbent products such as sanitary napkins, surgical bandages, disposable bed pads, and the like.
Disposable diapers are typically constructed of three components. Two of these components are a backing sheet, formed for example of a polyethylene film, and a facing layer that has its peripheral portions attached to the backing sheet. The facing layer is positioned next to the infant when the diaper is in use. The third component, which is contained between the first two components just described, is a cellulosic batt, formed for example of wood pulp fluff or cotton linters.
The facing layer of a known disposable diaper is typically a one-ply layer formed of rayon and wood pulp fibers, with a surfactant and a water repellent binder, such as an acrylic polymer, distributed through the layer of fibers in a specified manner. A balance of wettability is achieved in this way in order to control the flow of liquid through the facing layer in a desired manner. The facing layer is (1) not passing from outside the diaper through the layer into the cellulosic batt located in the mid-portion of the diaper, but (2) more water repellent than the cellulosic batt, so that urine that passes through the layer will remain in the batt rather than passing back out of the diaper through the facing layer.